Image formation employing electrophotographic systems is mainly performed by digital systems. In digital image formation systems, it is essential to use a toner of a minute particle diameter capable of achieving excellent fine line reproduction and high definition, as represented by visualization of images comprised of small dots, for example, at a level of 12,000 dpi (the number of dots per inch).
Further, in Patent Documents 1 and 2, disclosed as an example of production of such a small particle diameter toner is a production method in which toner raw materials such as polyester resins and the like are emulsify-dispersed in a water-based medium and resin particles in the resulting emulsified dispersion are aggregated to the desired toner size.
Further, known as an embodiment of the aforesaid digital image formation, is an image forming method of a print-on-demand system in which the required number of prints are carried out at the required occasions. Image formation utilizing the above system does not necessitate plate making which is performed in conventional printing, and makes it possible to readily achieve production of several hundred copies of publication, as well as production of direct mail and invitation cards while varying mailing addresses. Consequently, the above system is receiving attention as a promising image forming means replacing shortrun printing.
However, it has been noted that problems occur when image formation by the electrophotographic system is employed to produce mail and invitation cards while varying mailing addresses. The problems were that when images were formed on thick paper employed for invitation cards for wedding ceremonies, thick postcards, and gratitude cards for attending a funeral, it was not possible to achieve sufficient fixing. Specifically, in thick postcards and gratitude cards for attending a funeral, provided with a printed gray frame, fixing tends to not be sufficient within the gray frame, resulting in staining of the users' hands and other paper surfaces.
Further, when a toner image is formed on the surface of thick paper, an excessively large load, beyond comparison to that applied onto copy paper sheets, is applied to toner particles. As a result, toner particles tend to be crushed during image formation, whereby problems occur in which the paper surface is stained with powdered toner due to the crushing.
Thick paper such as the aforesaid thick postcards is one of the transfer media with high difficulty. However, in order to increase the use of electrophotography as an image forming means of the print-on-demand system, it is required that toner images are stably formed not only on plain paper developed for electrophotography as recording media, but also on printing paper. If this condition is not satisfied, electrophotography will not be accepted by printing industry.
For example, it is often viewed that a commuter is reading a paperback edition while holding the edition in one hand and hanging on to an overhead strap with the other hand. In such a situation, it is desired that the paper of the edition exhibits lubrication so that pages can be turned only by one hand, and at the same time, toner exhibits the fixing strength so that toner rubbed by friction does not result in staining of the paper surface and the text.
However, the slip property and fixing strength of toner images formed by the electrophotographic system are currently inferior to those of traditional printed matter. As a result, the aforesaid toner images have not been accepted by publishing institutions, resulting in its delayed use. The aforesaid inferiority has not been overcome even by employing toners disclosed in Patent Documents 1 and 2.
Further, toner which is prepared by aggregating resin particles tends to contain a relatively large amount of moisture, since the aggregation is performed in a water-based medium. When such a toner is employed, problems of toner blister are pronounced which is caused by release of toner from a toner image which is formed by generating air bubble formed in such a manner that moisture is evaporated by heat during fixing.
(1) Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2002-296839 (refer to paragraph 0011)
(2) Patent Document 2: JP-A No. 2002-351140 (refer to paragraph 0011)